The tip clearance effects on aero-elastic stability of axial compressor blades are investigated with two independent three-dimensional (3D) flutter prediction approaches: energy method and aero-elastic eigenvalue analysis. An axial compressor rotor which has encountered broken fault caused by flutter during the test rig and flight has been analyzed for five tip gap configurations. A consistent conclusion obtained by these two independent approaches shows the variation trend of aerodynamic damping is not monotonic, but aerodynamic damping at the least stable case shows a trend of first decrease and then increase with the rising of tip gap size, which is different from the results of other researchers and can be utilized to understand the conflict between the conclusions of different research work. Apart from the results of tip clearance effects on aero-elastic stability, the employed two methods have revealed the key factors involved in the flutter occurrence from a different perspective.

Particle damping (PD) is a passive mean of vibration control in which small metallic or ceramic particles are placed inside a cavity that attached to the primary structure at the place of high vibration amplitudes. The kinetic energy of the primary structure is dissipated by non-elastic impact and friction between particles and walls. This paper represents a series of experimental investigations of the effects of multi-unit particle dampers (MUPD) attached to a bracket structure under harmonic excitation and random excitation. As a platform to investigate the particle damping characteristics under extreme acceleration environments, the bracket structure was featured by an extremely high response on the top, and its maximum acceleration exceeds 50 times gravity acceleration when the bracket structure was subjected to resonance. This broad range of acceleration conditions was far beyond the scope concerned in most previous work. The experimental results show that for a small weight penalty (no more than 8.8%), multi-unit particle damper can reduce the resonance of the primary structure by more than 50%, whether under sinusoidal excitation or random excitation. And the response of the primary structure depends on the type of cavities and filled coefficient. Layering the cavity in the direction of the main vibration can improve the damping capacity of the multi-unit particle damper. And the damper with small particle size and large number of features is suitable for vibration reduction under high acceleration conditions.

Blade tip timing (BTT) measurement technology is more widely used to determine the vibrational stress of rotating blades and play an important role for blade service life prediction. The dynamic blade displacements can be measured by tip timing sensors, and then be converted to blade stress by the modal shape information from finite element method (FEM) analysis. However, there are always two uncertainties between the measured displacements by BTT and the modal shape by FEM analysis. First, the effective positions detected by sensors may shift from where they expected due to the deformation of the blade. This deviation may yield calibration factors with deceptions, which will present an inaccurate correlation for the blade stress level and the tip displacement. Second, when vibrating, blade tip would actually oscillate around the equilibrium position both in circumferential and axial direction, while the sensors can only detect the movements along the circumference direction and neglect the other. This causes the measured displacements to be different from the actual displacements. To deal with these two problems, a novel method based on the vibration amplitudes of blade tip along axial direction is proposed to identify the effective detected position. The vibration stress of the whole blade then can be determined by linking the modified displacements to the mode shape information from finite element (FE) predictions. This method is validated by a numerical BTT simulator, which is trying to simulate the actual testing process of BTT measurement. Both synchronous and asynchronous vibrations are discussed to illustrate the applicability of this method. Moreover, sensitivity analysis is performed to identify the uncertainties from the vibration amplitude and mode shape inaccuracies. Results demonstrate the great potential of the method for vibration stress determination.

In turbomachinery applications, thin-walled gears are cyclic symmetric structures and often subject to dynamic meshing loading which may result in high cycle fatigue (HCF) of the thin-walled gear. To avoid HCF failure, ring dampers are designed for gears to increase damping and reduce resonance amplitude. Ring dampers are installed in the groove. They are held in contact with the groove by normal pressure generated by interference or centrifugal force. Vibration energy is attenuated (converted to heat) by frictional force on the contact interface when the relative motion between ring dampers and gears takes place. In this article, a numerical method for the prediction of friction damping in thin-walled gears with ring dampers is proposed. The nonlinear damping due to the friction is expressed as equivalent mechanical damping in the form of vibration stress dependence. This method avoids the forced response analysis of nonlinear structures, thereby significantly reducing the time required for calculation. The validity of this numerical method is examined by a comparison with literature data. The method is applied to a thin-walled gear with a ring damper and the effect of design parameters on friction damping is studied. It is shown that the rotating speed, geometric size of ring dampers and friction coefficient significantly influence the damping performance.

The mistuning effects on the aero-elastic stability of axial compressor rotor blades have been investigated by employing aero-elastic eigenvalue method. An axial compressor rotor suffered from flutter failure has been analyzed with four intentional mistuning patterns and their sensitivities to random mistuning. Through a comparison of four intentional mistuning patterns, the mistuning mechanism of improving aero-elastic stability may be understood from the combining effects of frequency offset and the destruction of periodicity by the introduction of mistuning. Increasing the amount of alternate mistuning cannot provide any additional improving effects on the aero-elastic stability when the mistuning amount reaches a critical value. However, the further mistuning effects can be obtained by placing several groups (isolation zone) with additional frequency offset relative to the alternately mistuned system. The random mistuning can always improve the aero-elastic stability of the tuned system. However, for the intentionally mistuned system, the inclusion of random mistuning with a small level may weaken the improving effects of intentional mistuning and even makes the system unstable.